As discussed here
and previously, Simutrans-Experimental has now become Simutrans-Extended as of to-day, the 13th of February 2017.
To summarise: the game (including its long-term develoment aims) is exactly the same as what was Simutrans-Experimental. The change of name is intended to make clearer that Simutrans-Extended is a fork of Simutrans-Standard aimed at extending the basic game-play principally by adding realism. Some people had previously been confused by the old name, Simutrans-Experimental, which gave the impression that this was a branch for testing features for eventual inclusion into Simutrans-Standard.
The history of the name Simutrans-Experimental is that it was originally developed from late 2008 with the aim of adding one or two features to Simutrans rather than being a separate fork. The idea of a separate fork as the project has become developed only later when it emerged that there were different preferences as to how the game should play, with some users preferring the relative simplicity of Simutrans-Standard, and others preferring greater realism and complexity.